A REDUCTION IN THE NUMBER OF DIRECTIONALLY SELECTIVE NEURONS EXTENDS THE SPATIAL LIMIT FOR GLOBAL MOTION PERCEPTION

Dynamic random-dot targets were used to study neural mechanisms underlying motion perception. Performance of cats with severely reduced numbers of cortical directionally selective neurons (reduced DS) was compared to that of normal animals. We assessed the spatial properties of the residual motion mechanism by measuring direction discriminations at various dot displacements. At small displacements, reduced DS cats' motion integration thresholds for opposite direction discrimination were nearly normal. At larger displacements, their thresholds surpassed those of normal cats and their upper displacement limit (d(max)) was increased by 0.35 deg. The accuracy of direction discrimination was reduced at small displacements, but at larger displacements direction difference thresholds of reduced DS cats approached or surpassed those of normals. These data were compared to the performance of humans who showed an extension of d(max) for peripherally viewed targets. The data support the hypothesis that expansion in spatial scale of the motion mechanism may contribute to extension of d(max). Additional support for this hypothesis is provided by a modified direction discriminating line-element model. The model also suggests that changes in sampling of motion mechanisms in the reduced DS system may play a role.